Examples of gases containing oxides of nitrogen (hereinafter referred to an NO.sub.x) are exhaust gases from combustion apparatus such as boilers, nitric acid manufacturing plants and various metal treating processes.
In recent years, concern has grown over the so-called photochemical smog. One of the main causes of such photochemical smog is a large quantity of NO.sub.x present in the atmosphere. It is therefore desirable to reduce the quantity of NO.sub.x contained in such exhaust gases and/or to remove NO.sub.x from such exhaust gases.
In a combustion apparatus such as a boiler, for example, the NO.sub.x content in the exhaust gas has been reduced by use of burners and furnaces of improved design. These improved designs, however, are not very effective because they allow the reduction of NO.sub.x only within narrow limits of operation for both theoretical and economical reasons.
It is also well known in the art to employ the so-called wet type processes, for the removal of NO.sub.x from an exhaust gas, using an alkaline aqueous solution containing sodium hydroxide or sodium sulfite; an aqueous solution of potassium permanganate; an aqueous solution of hypochlorite or chlorite; or an aqueous solution of ferrous salt and sulfurous acid alkali salt (alkali sulfite).